Linux distributions or software installers) which allow the user to verify the file before installing. It is quite common to find hash values for download files on websites (e.g. If the digests differ, the data has changed in transit. When the message is received, the recipient calculates the digest from the received data and verifies that it matches with the one calculated by the sender. The digest is then sent alongside the message to the recipient. To verify integrity in practice using a hash function, the sender first calculates the digest for the message or document. Also, it is very hard to find two inputs that produce the same digest (collision resistance). Hash functions are also designed so that even a minute change in the input produces very different digest output. For instance, SHA256 hash function always produces 256-bit output. A hash function takes an arbitrary length data and produce a fixed sized digest for it. Common method to verify integrity is to use a hash function. To understand what makes a digital signature, the two requirements, integrity and authenticity, should be first examined separately. First part describes what is a digital signature and then the second part shows how to use OpenSSL sign and verify functions to work with signatures. This blog post describes how to use digital signatures with OpenSSL in practice. Digital signatures allow the recipient to verify both authenticity and integrity of the received document. It is needed for instance when distributing software packages and installers and when delivering firmware to an embedded device. The client program shown here connects over HTTPS to Google: /* compilation: gcc -o client client.Being able to verify that a piece of data originates from a trusted source (authenticity) and that it has not been altered in transit (integrity) is a common requirement in many use cases. Let’s start with a familiar example-accessing a web site with HTTPS-and use this example to pick apart the cryptographic pieces of interest. Command-line and code examples are one way to bring the main topics into focus together. The documentation for OpenSSL is spotty beyond the man pages, which become unwieldy given how big the OpenSSL toolkit is. Furthermore, calling OpenSSL command-line utilities begins with the term openssl. For example, OpenSSL functions often have SSL in the name even when TLS rather than SSL is in play. Nonetheless, it is common to refer to SSL/TLS as if they are one and the same protocol. TLSv1 and SSLv3 are alike, but not enough so to work together. SSL is versioned (e.g., SSLv2 and SSLv3), and in 1999 Transport Layer Security (TLS) emerged as a similar protocol based upon SSLv3. This feature is implemented with hash functions, which likewise come with the OpenSSL toolkit. For example, SSL supports message integrity, which assures that a received message is the same as the one sent. These two key SSL services, in turn, are tied to others that get less attention. Even if eavesdropper Eve intercepts an encrypted message from Alice to Bob (a man-in-the-middle attack), Eve finds it computationally infeasible to decrypt this message. This process safeguards network conversations. The receiver then decrypts each received message. Confidentiality: A sender encrypts messages before sending these over a channel.If Alice and Bob are to exchange messages over SSL, then each first authenticates the identity of the other. Peer authentication (aka mutual challenge): Each side of a connection authenticates the identity of the other side.The SSL protocol provides various security services, including two that are central in HTTPS: This protocol layer can sit atop HTTP, thereby providing the S for secure in HTTPS. Secure Socket Layer (SSL) is a cryptographic protocol that Netscape released in 1995. Let’s start with a review of the SSL in the OpenSSL name. You can find the code and command-line examples in a ZIP file from my website. The two articles in this series cover-collectively-cryptographic hashes, digital signatures, encryption and decryption, and digital certificates. The sample program for this article is in C, the source language for the OpenSSL libraries. (To install the most recent version of OpenSSL, see here.) OpenSSL utilities are available at the command line, and programs can call functions from the OpenSSL libraries. This article is the first of two on cryptography basics using OpenSSL, a production-grade library and toolkit popular on Linux and other systems.
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